A well-known electrically assisted bicycle is propelled with the help of a power generated by an electric motor in response to a pedaling force of a rider, as disclosed in JP-A-2000-53069.
An electrically assisted bicycle disclosed in JP-A-2000-53069 includes a magnetostrictive torque sensor for detecting a rider's pedaling force. The magnetostrictive torque sensor, which includes coils and magnetostrictive portions, is disposed along an outer circumference of a crankshaft.
The crankshaft is subjected to a torsional torque when the rider pedals the bicycle. The application of the torsional torque to the crankshaft causes a change in magnetic field in the magnetostrictive portions formed on an outer circumferential surface of the crankshaft. The change in magnetic field is detected by the coils, and a control unit drives an electric motor in response to the detection to produce an auxiliary torque to be transmitted via a sprocket and a chain to a rear wheel. The auxiliary torque is added to a pedaling torque exerted by the rider's feet on pedals of the bicycle, whereby the rider can propel the bicycle with a small pedaling force.
The electrically assisted bicycle may be propelled under various temperature conditions. For example, a temperature of an air may change during propulsion of the bicycle. This temperature change may affect the coils and the magnetostrictive portions in different amounts of time because the coils and the magnetostrictive portions are supported by separate members. That is, a difference in temperature may occur between the coils and the magnetostrictive portions. This difference in temperature causes an error in the pedaling torque detected by the magnetostrictive torque sensor. If a pedaling torque detected by the sensor is smaller than an actual pedaling torque exerted by the rider's feet on the pedals, an auxiliary torque produced by the electric motor is smaller than a desired auxiliary torque which should have been produced by the electric motor. When such a smaller torque is produced by the electric motor, the rider has to pedal the bicycle with a large pedaling force.